In a typical electro-photographic printing process, a photoreceptive member is sensitized by charging its surface to a substantially uniform potential. The charged portion of the photoreceptive member is exposed to a light image of an original image being reproduced. Exposure of the charged photoreceptive member selectively dissipates the charge in the irradiated areas to record an electrostatic latent image on the photoreceptive member. After the electrostatic latent image is recorded on the photoreceptive member, developer material is brought into contact with the image area of the photoreceptive member to develop the latent image.
Latent image development occurs as toner particles are removed from the developer material and adhere to the latent image. Developer material is generally comprised of toner particles and carrier granules. The toner particles adhere to the carrier granules by charge that is generated triboelectrically. As the developer comes into contact with the latent image, the toner particles are attracted by the charge of the latent image and migrate from the carrier granules to the latent image. This migration of toner particles to the latent image forms a toner powder image on the photoreceptive member. The toner powder image is then transferred from the photoreceptive member to a copy sheet. The toner particles are heated to permanently affix the powder image to the copy sheet.
In most two component development stations, the developer material is mechanically stressed as it is mixed by one or more augers and travels on the magnetic surface roller. These mechanical stresses help to triboelectrically charge the developer so the toner particles adhere to the carrier granules. The toner is not always removed from the developer at a constant rate, however, because some images require more toner for development than other images. Thus, toner may be charged for used and provided to the magnetic roller, but remain on the roller as the latent image passes. That is, the portion of the latent image closest to the developer on the magnetic roller does not have sufficient charge to attract the toner. Consequently, the developer is returned to the developer sump for later use.
The developer returned to the sump is again subjected to mixing. After some time, the mechanical stress on the toner particles causes their surface additives to impact into the toner particles and the toner can also impact onto the carrier granules. Surface additives are included to control charge and to lower toner adhesion to the photoreceptor and other surfaces for improvement of transfer efficiency and image uniformity. Continuing this impact eventually degrades the usefulness of the toner surface additives and the ability of the toner particles to hold a charge sufficient for good image quality. Electro-photographic machines include components and processes for purging these toner particles from the system. The removed toner, however, is toner that was not used to generate images. This toner must be replaced by fresh toner, some of which is eventually unused and removed from the machine. Thus, depending on the content of the images to be developed by the machine, the developer provided in a developer supply may not all be efficiently used. Moreover, the number of quality images produced by a developer supply is reduced by the loss of toner that was not used to generate images.
One way of addressing this issue is to drive the mixing augers and magnetic rollers at a speed that does not overly stress the developer material. As noted above, however, the demand for toner in an image varies from image to image. Consequently, the development station needs to charge adequately a sufficient amount of toner to meet the varying requirements for developing the latent images on the photoreceptive member. Currently, the waste of some toner is deemed an acceptable tradeoff for providing sufficient toner for developing images having a variable amount of content.